An introduction to MEG Lecture 1

1. An introduction to MEG Lecture 1 Matt Brookes

2. What is Magnetoencephalography? Cellular currents produce magnetic fields Aim of MEG: To detect these magnetic fields and use them to reconstruct the electrical neuronal activity in the brain

3. What is Magnetoencephalography? Head is placed in a helmet surrounded by ~300 field detectors Spatial topography of the magnetic field measured Field Detectors Dewar filled with liquid helium Subject

4. What is Magnetoencephalography? 275 channel MEG scanner at the SPMMRC

5. Neural generation of magnetic fields Schematic Illustration of a neuron

6. Neural generation of magnetic fields Pyramidal (left) and stellate (right) neurons Symmetric distribution of dendrites in stellate cells means that the magnetic fields cancel out Fields in MEG therefore due to pyramidal cells, not stellate cells

7. Neural generation of magnetic fields • Post synaptic currents • Caused by chemical interaction at a synapse • Termination of an action potential from pre-synaptic cell causes neurotransmitter release • Neurotransmitter causes opening of ion channels on post synaptic cell wall • Ions rush into the cell and pass down the dendrites towards the cell body • Result – Dendritic current • Whole process lasts a few tens of milliseconds

8. Neural generation of magnetic fields • Axonal currents • Dendritic currents from excitatory synapses increase electrical potential at the cell body • When potential at the axon hillock reaches a threshold value (~ -40mV), an action potential is sent down the axon • Axon is insulated with a myelin sheath • Action potential mediated by leading edge of depolarisation • Time scale of an action potential is ~1ms

9. Neural generation of magnetic fields Dendritic current / post synaptic potential Action potential Acts as a current dipole Dipole moment ~25fAm Magnetic fields falls off as… Acts as two back to back current dipoles each with magnitude ~100fAm But magnetic fields falls off as…

10. The forward problem Given a known current distribution in the brain, can we compute the magnetic field distribution outside the brain?

11. The inverse problem Given a known magnetic field distribution outside the head, can we compute the current distribution in the brain?

12. An introduction to MEG Lecture 2 The MEG forward and inverse problems

13. Radial Dipoles Actual detection probability for a whole head (151 channel) MEG scanner. Notice that radial dipoles cannot be detected, however a large percentage of the cortex is detectable.

14. Dipolar field patterns Left – measured dipolar field pattern representing the neuromagnetic response to a somatosensory stimulus Right – schematic showing dipolar magnetic field

15. Dipolar field patterns Measured magnetic fields in response to an auditory stimulus

16. Inverse Solution fMRI MEG

17. Inverse Solution

18. An introduction to MEG Lecture 3 Detectable neuromagnetic effects

19. Brain rhythms Hans Berger – 1929 – Discovered that electrical potentials can be recorded from the scalp surface. These potentials are directly reflective of current flow in neurons in the cerebral cortex Discovered the alpha rhythm

20. Brain rhythms

21. Induced and evoked effects Two types of MEG signal • Time-locked and Phase-locked evoked responses REST STIM REST STIM REST • Time-locked and non-phase-locked induced oscillatory responses REST STIM REST STIM REST

22. Neuromagnetic responses to visual stimulation β-band ERS (15-30Hz) Ŧ>1.2 7T BOLD T>6 β-band ERD (15-30Hz) Ŧ>1.2 3T BOLD T>5.5 VEP Ŧ>5 γ-band ERS (60-80Hz) Ŧ>4

23. Neuromagnetic responses to visual stimulation